Aperture fluorescent lamp for copying machines

ABSTRACT

An aperture fluorescent lamp wherein luminous flux is radiated uniformly along the aperture. In a first embodiment, a variable area aperture is provided. In the second embodiment, a variable area reflector is formed on the outside surface of the lamp.

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tates Pater mm [4 1 0a. 23, 1973 [54] APERTUIRE FLUORESCENT LAMP FOR2,407,379 9/1946 Morehouse 313/109 CO NG MACHINES 3,225,241 12/1965Spencer et al. 313/109 3,275,872 9/1966 Chernin et a1. 313/109 [75]inventor: George Thomas Bauer, Rochester,

Primary Examiner-Palmer C. Demeo [73] Asslgnee' Xerox CorporationRochester Attorney-James J. Ralabate, John E. Beck and Irving [22]Filed: Dec. 241, 1969 Keschner Appl. N0.: 887,846

US. Cl 313/109, 313/113, 313/220 lint. C1 H0lj 61/35, HOlj 61/42 Fieldof Search ..313/109,113,117,

References Cited UNITED STATES PATENTS Ill 1938 Randall et a1 313/113 X[57] ABSTRACT An aperture fluorescent lamp wherein luminous flux isradiated uniformly along the aperture. in a first embodiment, a variablearea aperture is provided. In the second embodiment, a variable areareflector is formed on the outside surface of the lamp.

2 Claims, 8 Drawing Figures PAIENIEnnm 23 ms 3. 767' 956 sum 1 orINVENTOR. GEORGE T. BAUER ATTORNEY AIERTURE FLUORESCENT LAMP FOR COPYINGMACHINES BACKGROUND OF THE INVENTION In the xerographic process asdescribed in US. Pat. No. 2,297,691, a base plate of relatively lowelectrical resistance such as metal, etc., having a photoconductiveinsulating surface coated thereon is electrostatically charged in thedark. The charged coating is then exposed to a light image. The chargesleak off rapidly in the base plate in proportion to the intensity oflight to which any given area is exposed, the charge being substantiallyretained in non-exposed areas. After exposure, the coating is contactedwith electrostatic materials which adhere to the remaining charges toform a powder image corresponding to the latent electrostatic latentimage remaining after exposure. The powder image then can be transferredto a sheet of transfer material resulting in a positive or negativeprint, as the case may be. Since dissipation of the surfaceelectrostatic charge is proportional to the intensity of the impingingradiation, light sources of uniform and sufficient intensity must beprovided so that the photoconductive insulator can be properly exposed.

It is generally known that the luminous intensity from aperturefluorescent lamps decreases sharply from the center towards the endsthereof. This lack of uniformity along the length of the lamp is adisadvantage when the lamps are used in xerography wherein it is desiredto have substantial uniform illuminance across the width of the materialto be copied. By limiting the aperture, or slot, to a length which isless than that of the positive column, or distance between thevthermionic electrodes, the prior art provided an aperture fluorescentlamp which produced a substantially uniform luminous intensity along theaxis of the aperture. However, restricting the useful length of thelamps to an aperture of less than the positive column of the lamp hastwo disadvantages associated therewith. Light produced within the lampin the areas corresponding to the non-aperture portions of the lamp isnot completely utilized, decreasing the efficiency of the lamp. Inaddition, the shortened length of the aperture effectively limits thewidth of the material which may be copied.

SUMMARY OF THE INVENTION The present invention provides an aperturefluorescent lamp having a uniform luminous intensity along its apertureaxis, and, in particular, wherein the uniform luminous intensity isobtained along the entire positive column of the lamp. In a firstembodiment, the aperture is wider towards the ends of the glass tubeenvelope. In a second embodiment, the outside of the glass tubeenvelope, except for the aperture, is covered with a reflecting memberwhose surface area increases towards the tube ends. Both embodimentsutilize the entire positive column of the lamp and uniform luminousintensity can be achieved without a significant loss in lamp efficiency.

It is an object of the present invention to provide an apertureflurorescent lamp which produces a uniform luminous intensity along theaperture axis.

It is a further object of the present invention to provide an aperturefluorescent lamp which produces uniform luminous intensity along theaperture axis wherein the length of the aperture is equal to thepositive column of the lamp.

It is still a further object of the present invention to provide anaperture fluorescent lamp wherein the length of the aperture is equal tothe positive column and the width of the aperture varies along theaperture length.

It is a further object of the present invention to provide an aperturefluorescent lamp wherein the aperture is equal to the positive columnand wherein a reflecting layer of varying surface area is formed on theoutside of the glass envelope except for the aperture.

DESCRIPTION OF THE DRAWINGS For a better understanding of the inventionas well as other objects and further features thereof, reference is madeto the following detailed description which is to be read in conjunctionwith the accompanying drawings wherein:

FIG. 1 shows an aperture fluorescent lamp with a shortened aperture asfound in the prior art;

FIG. 2 is a cross-sectional view along line 22 of the lamp shown in FIG.1;

FIG. 3 shows an aperature fluorescent lamp with the aperture widthincreasing towards the ends of the tube envelope in accordance with theteachings of the present invention;

- FIG. 4 is a cross-sectional view along line 44 of the lamp shown inFIG. 3;

FIG. 5 shows another embodiment of the aperture fluorescent lamp of thepresent invention which illustrates a variable width aperture;

FIG. 6 is a cross-sectional view along line 5-5 of the lamp shown inFIG. 5;

FIG. 7 shows an additional embodiment of the aperture fluorescent lampof the present invention which utilizes a reflecting layer whose surfacearea increases towards the ends of the tube envelope; and

FIG. 8 is a cross-sectional view along line 8-8 of the lamp shown inFIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a prior art aperture fluorescent lamp, the construction ofwhich is similar to the lamp of the present invention, the improvementsthereover which constitute the present invention being describedhereinafter with reference to FIGS. 3 through 8. The lamp comprises anelongated glass tube 10 forming the envelope into the ends of which aresealed a pair of electrodes 12 and 14. The electrodes may be of thethermionic type, each comprising a tungsten filament coated withelectron emitting material consisting of alkaline earth oxides and thesupport and lead-in wires 16 and 18 sealed through the usual stern pressand connected to terminal pins 20 and 22 of a base 24. The envelope isfilled with inert gas, for instance argon or a mixture of argon withanother rare gas as helium, at a pressure of a few millimeters ofmercury with sufficient mercury to provide a vapor pressure of a fewmicrons in operation. It should be noted that the present invention, asdescribed hereinbelow, may be utilized in high pressure discharge lamps.A narrow aperture, or slot, 30 is provided by scraping out thereflective and phosphor coatings over a minor portion of the interiorperiphery, for instance over an arc of 60. The actinic energy output ofaperture lamps is dependent on, among other factors, the geometricalconfiguration of the aperture. The amount of luminous flux emittedthrough the aperture is proportional to the area of the aperture. Asdiscussed hereinabove, it is known that the luminous intensity of anaperture fluorescent lamp along the length of the lamp decreases towardsthe ends. Therefore, the aperture in conventional lamps only extends fora portion of its length as shown in FIG. 1.

Referring now to FIG. 2, there is shown a crosssectional view along line2-2 of FIG. 1. Reflective coating 26 is applied to the inside surface ofthe glass envelope over the major portion of the periphery and aphosphor coating 28 is applied thereover. The reflective coating maycomprise powdered materials such as titanum dioxide having a particlesize less than one micron, magnesium oxide, zinc oxide, zirconia, ormetals such as aluminum or silver.

The aperture shown in FIG. 2 is clear of the reflective and phosphorcoating. It should be obvious that the invention as hereinafterdescribed is applicable to a reflector fluorescent lamp with thephosphor coating applied to the aperture. The phosphor coating 28 may bemade thick enough to reflect into the envelope a large portion of thelight emitted from the phosphor coating, thereby eliminating thenecessity of a separate reflective coating. The phosphor coating 28 maycomprise calcium halophosphate activated with manganese and antimony orany other suitable fluorescent lamp phosphor. The methods of applyingthe reflective coating and phosphors to the tube walls and forming aphosphor or clear aperture is well known and will not be describedherein.

FIG. 3 shows one possible lamp configuration wherein a variable areaaperture is formed. A variable area aperture is provided whereby thetotal length, or positive column of the aperture lamp is utilized while,at the same time, providing a substantially uniform luminous intensityalong the total length of the aperture fluorescent lamp. Portion 40 ofthe aperture is selected to correspond to the length of aperture 30 asshown in FIG. 1 such that the decrease in lamp brightness from thecenter of the lamp towards the ends normally would become pronounced ifthe length of aperture 40 was extended without an attendant increase inaperture area. However, in accordance with the teachings of the presentinvention, the aperture changes in area towards the ends of the lamp,corresponding to the step-shaped portions 42 of the aperture. Portion 42may be formed in the same manner as apertures 30 or 40, discussedhereinabove. The increased aperture area towards the ends of the lamp(portions 42)increases the amount of luminous flux radiated thereat,since a greater area of the phosphor coating 28 is seen than alongaperture portion 40. In other words, the amount of luminous flux emittedby the phosphor coating 28 through an aperture is proportional to thearea of the aperture.

FIG. 4 is a cross-sectional view along line 4-4 of FIG. 3 wherein thedotted lines indicate the stepshaped aperture portions 42.

Referring now to FIG. 5, there is shown another embodiment of the novelaperture lamp of the present invention. The output of aperture portion50, as defined by dashed lines 54 and 56, is chosen to providesubstantially uniform luminous intensity, as discussed with reference toaperture 30 and aperture portion 40 hereinabove. The remaining portionsof the aperture shown in FIG. 5 comprise curved portions 52. Thisaperture configuration is designed to increase the luminous intensity ata rate equal to the rate of decrease in the intensity which wouldnormally occur if the aperture area was constant along the lamp length.

The variable area portions of the apertures as illustrated in FIGS. 3and 5 are illustrative of the type of configurations which may beutilized. Many other variable area aperture configurations may beprovided and still be within the purview of the present invention.

FIG. 6 is a cross-sectional view along line 6-6 of FIG. 5 showing theeffect of the changed aperture geometry.

Referring now to FIG. 7, another embodiment of the present invention isshown. In this embodiment, reflecting layer 26, shown in the priorfigures, is omitted. It is known that only a portion of the luminousflux generated inside of the lamp is radiated through the aperture. Someother portion of the luminous flux is radiated through the glassenvelope. It has been determined that when a reflecting layer covers theouter surface of the envelope the flux radiated through the aperture isincreased. Substantially uniform luminous flux is radiated through theaperture by providing a variable area reflecting layer which covers theoutside of the lamp and which widens towards the lamp ends. It isassumed that the thickness of phosphor layer 28 is chosen to permit aportion of the luminous flux generated in the lamp to pass therethrough.The reflecting layer may comprise aluminum, silver or any other suitablelight reflecting material and is deposited on the lamp surface by knowndeposition techniques, such as by evaporation. The reflecting layer mayalso comprise aluminum foil.

The width of reflecting layer 62 formed on the surface of envelope [0varies in proportion to the normal fall-off in luminous intensity alongthe length of the aperture. It should be noted that reflecting layer 62also provides for better starting and cooling of the lamps. Bettercooling increases the lifetime and efficiency of the lamps since thechemical processes that cause lamp deterioration slows at lowertemperatures.

An alternate embodiment to the embodiment shown in FIG. 7 would includethe reflecting layer 26 since reflectors are normally not perfect i.e. aportion of the luminous flux generated in the lamp will pass throughreflecting layer 26 and impinge upon reflecting layer 62.

FIG. 8 is a cross-sectional view along line 88 of FIG. 7 showing thereflecting layer 62.

The embodiment shown in FIGS. 3, 5, and 7 are illustrative of thevarious types of variable area aperture and reflecting layerconfigurations that may be utilized in the present invention and variousother configurations may also be utilized.

What is claimed is:

1. An aperture fluorescent lamp comprising:

an elongated tubular glass envelope having electrodes sealed into itsopposite ends and containing an ionizable medium therein;

a coating applied to the interior surface of said glass envelope, saidcoating comprising a phosphor layer extending over the entire interiorof said envelope except for a narrow aperture extending alongsubstantially the whole length of said envelope; and

a reflecting member formed on the exterior surface of said glassenvelope except for the aperture, the surface area of said reflectingmember being greatest towards the ends of said envelope wherebysubstantially uniform luminous intensity is achieved along the wholelength of said aperture.

6 extending along substantially the whole length of said envelope, thesurface area-of said reflecting member being greatest towards the endsof said envelope whereby substantially uniform luminous intensity isachieved along the whole length of said aperture.

1. An aperture fluorescent lamp comprising: an elongated tubular glassenvelope having electrodes sealed into its opposite ends and containingan ionizable medium therein; a coating applied to the interior surfaceof said glass envelope, said coating comprising a phosphor layerextending over the entire interior of said envelope except for a narrowaperture extending along substantially the whole length of saidenvelope; and a reflecting member formed on the exterior surface of saidglass envelope except for the aperture, the surface area of saidreflecting member being greatest towards the ends of said envelopewhereby substantially uniform luminous intensity is achieved along thewhole length of said aperture.
 2. An aperture flourescent lampcomprising: an elongated tubular glass envelope having electrodes sealedinto its opposite ends and containing an ionizable medium therein; aphosphor coating extending over the entire interior of said envelope;and a reflecting member formed on the exterior surface of said glassenvelope except for a narrow aperture extending along substantially thewhole length of said envelope, the surface area of said reflectingmember being greatest towards the ends of said envelope wherebysubstantially uniform luminous intensity is achieved along the wholelength of said aperture.